1. Field of the Invention
The invention concerns a projection exposure apparatus with an illumination system and a projection objective as well as a projection objective in a projection exposure apparatus for wavelengths ≦193 nm.
In order to be able to further reduce the structural widths of electronic components, particularly in the submicron range, it is necessary to reduce the wavelengths of the light utilized for microlithography. Lithography with very deep UV radiation, so called VUV (Very deep UV) lithography or with soft x-ray radiation, so-called EUV (extreme UV) lithography, is conceivable at wavelengths smaller than 193 nm, for example.
2. Description of the Prior Art
An illumination system for a lithographic device, which uses EUV radiation, has been made known from U.S. Pat. No. 5,339,346. For uniform illumination in the reticle plane and filling of the pupil, U.S. Pat. No. 5,339,346 proposes a condenser, which is constructed as a collector lens and comprises at least 4 pairs of mirror facets, which are arranged symmetrically. A plasma light source is used as the light source.
In U.S. Pat. No. 5,737,137, an illumination system with a plasma light source comprising a condenser mirror is shown, in which an illumination of a mask or a reticle to be illuminated is achieved by means of spherical mirrors.
U.S. Pat. No. 5,361,292 shows an illumination system, in which a plasma light source is provided, and the point plasma light source is imaged in an annular illuminated surface by means of a condenser, which has five aspherical mirrors arranged off-center.
From U.S. Pat. No. 5,581,605, an illumination system has been made known, in which a photon beam is split into a multiple number of secondary light sources by means of a plate with concave raster elements. In this way, a homogeneous or uniform illumination is achieved in the reticle plane. The imaging of the reticle on the wafer to be exposed is produced by means of conventional reduction optics. A gridded mirror is precisely provided with equally curved elements in the illumination beam path.
From U.S. Pat. No. 5,353,322 a lens system for an X-Ray projection lithography camera having a source of X-Ray radiation, a wafer and a mask to be imaged on the wafer has been made know. According to U.S. Pat. No. 5,353,322 a chief ray, which is also called a principle ray, of the radiation incident on the mask is inclined away from the optical axis of the lens system in a direction from the source toward the mask. Whereas in U.S. Pat. No. 5,353,322 in principle a projection exposure apparatus has been made known, the projection exposure apparatus made known from U.S. Pat. No. 5,353,322 does not show how to illuminate the mask in the reticle plane in a homogenous manner.
From EP 0 939 341 A2 an illumination system and an exposure apparatus for wavelengths ≦193 nm has been made known with an optical integrator divided into raster elements for homogenous illumination of a field in an image plane. The raster elements are of arcuate shape as the field formed in the image plane. To illuminate the surface over the arcuate illumination field in an overlapping manner in the image plane, the illumination system of the projection exposure apparatus shown in EP 0 939 341 A2 further includes a condenser optic.
The projection lens shown in the U.S. Pat. No. 5,353,322 is a three mirror projection objective. A disadvantage of the projection objective shown in U.S. Pat. No. 5,353,322 is the small numerical aperture.
From U.S. Pat. No. 5,686,728 a six mirror projection objective is known. The projection objective disclosed therein is used only for UV-light with wavelengths in the region 100-300 nm. The mirrors of this projection objective have a very high asphericity of ±50 μm as well as very large angles of incidence of approximately 38°. Such asphericties and angles of incidence are not practicable for EUV. The aperture stop in the six-mirror objective known from U.S. Pat. No. 5,686,728 is situated between the second mirror and the third mirror. Due to the small distance of 200.525 mm between the first mirror and the second mirror the aperture stop can be varied in its position between the first and the second mirror only within a small range. Thus, the correction of telecentricity error, coma or astigmatism by shifting the position of the aperture stop is possible only within a small range.
The contents of the above-mentioned patents are incorporated by reference.
None of the aforementioned documents of the state of the art describe a projection objective which allows for a correction of telecentricity errors as well as coma and astigmatism in a broad range. Furthermore none of the references cited above shows a projection exposure apparatus with a homogenous illumination of a field in the image plane of the projection exposure apparatus and a high transmission of the light intensity entering the projection exposure apparatus.